Porous carbon sheet, gas diffusion electrode, fuel cell, liquid electrolysis device, redox flow battery, and moving body

Abstract

A porous carbon sheet according to the present invention has a water sliding angle of 0° to 30°, inclusive, on at least one surface thereof, and the fluorine / carbon intensity ratio of said surface is 0.10 to 0.30, inclusive.

Description

Porous carbon sheets, gas diffusion electrodes, fuel cells, liquid electrolytic devices, redox flow batteries, and mobile devices

[0001] The present invention relates to porous carbon sheets used in fuel cells, liquid electrolytic devices, and redox flow batteries, and more particularly to porous carbon sheets suitable for solid polymer fuel cells used as power sources for mobile devices such as fuel cell vehicles and ships.

[0002] Fuel cells are a type of power generation device that extracts electrical energy by electrochemically oxidizing fuels such as hydrogen and methanol, and have recently attracted attention as a clean energy source. Among them, polymer electrolyte fuel cells have a low standard operating temperature of around 100°C and high energy density, making them promising for a wide range of applications as power generation devices for relatively small-scale distributed power generation facilities, as well as for mobile devices such as fuel cell vehicles and ships.

[0003] The basic structure of a polymer electrolyte fuel cell consists of a polymer electrolyte membrane, catalyst layers formed on both sides of the polymer electrolyte membrane, gas diffusion electrodes formed outside the catalyst layers to diffuse fuel gas / oxidizing gas, and two separators that sandwich them.

[0004] Gas diffusion electrodes require high gas diffusivity to diffuse the gas supplied from the separator to the catalyst, high drainage to discharge water generated by the electrochemical reaction back to the separator, and high conductivity to extract the generated current. As a material that possesses all of the above characteristics, porous carbon sheets are generally used as gas diffusion electrodes. Specific examples of porous carbon sheets include conductive porous substrates such as carbon felt, carbon paper, and carbon fiber fabrics made from carbon fibers. Among these, carbon paper, which is a substrate obtained by binding a carbon fiber paper body with carbonized resin, is preferred in terms of mechanical strength and is generally used because it has excellent properties for absorbing dimensional changes in the thickness direction due to the swelling and shrinkage of the electrolyte membrane, i.e., "springiness".

[0005] However, because carbon paper has a structure in which carbon fibers are bound together with a binder, electricity needs to pass between multiple binders and carbon fibers for conductivity in the thickness direction, resulting in the problem of low conductivity in the thickness direction, and the binder also hindering gas diffusion. Improving the conductivity and gas diffusion properties of the gas diffusion layer is also a challenge in order to improve the performance of fuel cells, so studies are being conducted on using carbon fiber fabrics, which have excellent conductivity and gas diffusion properties, as a substitute for carbon paper in the gas diffusion layer.

[0006] For example, Patent Document 1 describes a technique for producing a carbon fiber fabric with minimal thickness variation and appropriate rigidity by first treating acrylic spun yarn as flame-resistant fibers, then weaving the fabric, pressing it to carbonize or graphitize it, and finally applying a binder.

[0007] Furthermore, Patent Document 2 describes a technique for producing a carbon fiber fabric of a thickness suitable for a gas diffusion layer by weaving thin acrylic fibers and then flame-retarding and carbonizing the fabric itself.

[0008] Japanese Patent Publication No. 2004-100102 Japanese Patent Publication No. 2019-71292

[0009] However, the carbon fiber fabric disclosed in Patent Document 1 is insufficient to reduce contact resistance with CCM (Catalyst Coated Membrane) and separators. In contrast, the carbon fiber fabric disclosed in Patent Document 2 can be made into a thin film and its resistance is improved, but it has the problem that its springiness decreases when it is made thin.

[0010] Therefore, the present invention has been made in view of the above, and aims to provide a porous carbon sheet, a gas diffusion electrode, a fuel cell, a liquid electrolytic device, a redox flow battery, and a mobile body that are excellent in springiness, drainage, and gas diffusion.

[0011] To solve the above problems, the present invention employs the following means: [1] A porous carbon sheet having a water sliding angle of 0 degrees or more and 30 degrees or less on at least one surface, and a fluorine / carbon strength ratio of 0.10 or more and 0.30 or less.

[0012] [2] The porous carbon sheet according to [1], wherein the porous carbon sheet is a carbon fiber felt or a carbon fiber fabric.

[0013] [3] A porous carbon sheet according to [1], comprising carbon fibers having a diameter of 3 μm or more and 10 μm or less.

[0014] [4] The porous carbon sheet according to [1], wherein the porous carbon sheet is made of a carbon fiber fabric and includes spun yarn in which the number of carbon fibers contained in the cross-section is 50 or more and 150 or less.

[0015] [5] The porous carbon sheet according to [1], wherein the porous carbon sheet is made of a carbon fiber fabric and the opening ratio of the carbon fiber fabric is 5% or more and 75% or less.

[0016] [6] The porous carbon sheet according to [1], comprising 0.1 parts by mass or more and 3 parts by mass or less of fluororesin per 100 parts by mass of the porous carbon sheet.

[0017] A gas diffusion electrode comprising a microporous layer composed of a carbon-based filler and a fluororesin formed on one side of the porous carbon sheet described in [7] [1].

[0018] [8] The gas diffusion electrode according to [7], characterized in that the amount of penetration, which is the ratio of the thickness of the portion of the microporous layer that has penetrated into the porous carbon sheet, is 10% or more and 100% or less, when the thickness of the porous carbon sheet, defined as the distance between the microporous layer side surface of the porous carbon sheet and the porous carbon side surface of the porous carbon sheet, is 100%.

[0019] [9] The gas diffusion electrode according to [7], wherein, in the range of pore diameters from 0.03 μm to 1.00 μm, the diameter of the pore with the largest volume (peak diameter) is in the range of 0.10 μm to 1.00 μm.

[0020]

[10] Air permeability within the plane is 100 μm ３ The gas diffusion electrode described above [7].

[0021] A fuel cell having a gas diffusion electrode as described in any of

[11] , [7], to

[10] .

[0022] A liquid electrolytic apparatus having a gas diffusion electrode as described in any of

[12] , [7], to

[10] .

[0023] A redox flow battery having a gas diffusion electrode as described in any of

[13] , [7], to

[10] .

[0024] A mobile vehicle equipped with the fuel cell described in

[14] and

[11] .

[0025] According to the present invention, a porous carbon sheet with excellent springiness, drainage, and gas diffusion properties can be obtained.

[0026] Figure 1 is a schematic diagram showing an example of a cross-section of a gas diffusion electrode according to an embodiment of the present invention.

[0027] The following describes in detail an embodiment of the porous carbon sheet according to the present invention, based on the drawings. However, the present invention is not limited to this embodiment. Furthermore, the individual embodiments of the present invention are not independent but can be combined and implemented as appropriate.

[0028] (Embodiment) The porous carbon sheet according to the present invention is a constituent material of a gas diffusion layer, which is a layer for diffusing fuel gas / oxidizing gas, and is made of carbon fiber, for example. A gas diffusion electrode is formed by forming a microporous layer composed of a carbon-based filler and a fluororesin on one side of this porous carbon sheet. Here, the gas diffusion electrode is formed on the outside of a polymer electrolyte membrane having catalyst layers on both sides in a battery.

[0029] Figure 1 is a schematic diagram showing an example of a cross-section of a gas diffusion electrode according to an embodiment of the present invention. The gas diffusion electrode consists of a microporous layer 1 formed using a carbon-based filler and a fluororesin, and a carbon fiber fabric 2 in which a plurality of carbon fibers 2a are woven, and is formed in a structure in which a part of the microporous layer 1 is impregnated into the carbon fiber fabric 2.

[0030] The porous carbon sheet of the present invention is preferably a carbon fiber felt or a carbon fiber fabric. Since carbon fiber felt or carbon fiber fabric does not require a binder to bond the carbon fibers together, it has a flexible structure. By applying a small amount of water-repellent resin, the porous carbon sheet is reinforced and its springiness is improved. In addition, because carbon fiber felt or carbon fiber fabric has a rough surface, high water repellency can be obtained with only a slight water-repellent treatment.

[0031] Carbon fiber fabrics are composed of warp and weft threads made of carbon fibers. The weaving method of carbon fiber fabrics may be plain weave, twill weave, or other structures. For fuel cell applications, a thin thickness of carbon fiber fabric is preferable, and therefore, it is preferable that the diameter of the carbon fibers constituting the carbon fiber fabric is also fine. In order to make the carbon fibers fine, it is preferable to use fine-diameter acrylic fiber spun yarn as the starting material. The thickness of the spun yarn is expressed in metric count as 1 / 50 Nm or less, preferably 1 / 100 Nm or less, and more preferably 1 / 200 Nm or less. By using spun yarn with a thickness of 1 / 50 Nm or less in metric count, carbon fiber spun yarn with a thickness of 1 / 70 Nm or less can be obtained after firing.

[0032] In this invention, the diameter of the carbon fibers constituting the porous carbon sheet is preferably 3 μm or more and 10 μm or less. A carbon fiber diameter of 10 μm or less allows for a thinner porous carbon sheet, thus providing a porous carbon sheet suitable for fuel cell applications. If the carbon fiber diameter is less than 3 μm, the strength and productivity of the porous carbon sheet may deteriorate.

[0033] When using a carbon fiber fabric in the porous carbon sheet of the present invention, the warp and weft threads constituting the carbon fiber fabric are preferably spun carbon fiber yarns, and it is preferable that the cross-section of each yarn contains 50 to 250 carbon fibers. By having 250 or fewer carbon fibers, more preferably 150 or fewer, in the warp and weft spun yarns, the carbon fiber fabric can be made thinner. If the number of carbon fibers in the warp or weft spun yarns is less than 50, the strength and productivity of the carbon fiber fabric may deteriorate.

[0034] When a carbon fiber woven fabric is used for the porous carbon sheet of the present invention, the carbon fiber woven fabric preferably has an opening ratio of 5% or more and 75% or less. An opening ratio of 5% or more for the carbon fiber woven fabric is preferable because a carbon fiber woven fabric excellent in gas diffusibility in the direction perpendicular to the plane can be obtained. Further, when the opening ratio of the carbon fiber woven fabric exceeds 75%, the strength and productivity of the carbon fiber woven fabric may deteriorate. Here, the opening ratio (%) of the carbon fiber woven fabric refers to a value defined by the following formula (1) when the warp pitch is Tp, the weft pitch is Yp, the warp width is Tw, and the weft width is Yw. Opening ratio [%]=(Tp−Tw)×(Yp−Yw) / Tp / Yp×100 ...(1)

[0035] Since the porous carbon sheet is used as an electrode under a pressurized state, it is preferable that the thickness when pressurized at a surface pressure of 1 MPa is 30 μm or more and 150 μm or less. A thickness of 150 μm or less when pressurized at a surface pressure of 1 MPa is preferable because it is excellent in conductivity and can suppress the size of a fuel cell unit. On the other hand, when the thickness is less than 30 μm, the strength of the porous carbon sheet and the in-plane gas diffusibility deteriorate.

[0036] The porous carbon sheet of the present invention is subjected to water-repellent treatment by applying a water-repellent resin to a carbon fiber woven fabric. As the water-repellent resin, a fluororesin can be used, and examples thereof include PTFE (polytetrafluoroethylene) (e.g., "Teflon (registered trademark)"), FEP (tetrafluoroethylene-hexafluoropropylene copolymer), PFA (perfluoroalkoxy fluororesin), ETFE (ethylene-tetrafluoroethylene copolymer), PVDF (polyvinylidene fluoride), PVF (polyvinyl fluoride), and the like. PTFE or FEP that exhibits strong water repellency is preferable.

[0037] The adhesion of the water-repellent resin reinforces the fiber bundles of the porous carbon sheet and improves spring properties, but if the amount of the water-repellent resin is too large, the porous carbon sheet becomes less likely to be crushed, and the spring properties deteriorate. In addition, since the porous carbon sheet of the present invention uses a carbon fiber woven fabric composed of fine-diameter carbon fiber yarns, high water repellency can be obtained only by applying a small amount of water-repellent resin, and both spring properties and water repellency can be achieved.

[0038] The amount of the water-repellent resin is preferably 0.1 parts by mass or more and 3 parts by mass or less based on 100 parts by mass of the porous carbon sheet, and is adjusted such that the fluorine / carbon intensity ratio on at least one surface of the porous carbon sheet is 0.10 or more and 0.30 or less. In this case, if the fluorine / carbon intensity ratio is less than 0.10, water repellency is insufficient and drainage performance decreases, and if it exceeds 0.30, electrical conductivity decreases. Further, a fluorine / carbon intensity ratio of 0.25 or less is more preferable because a porous carbon sheet excellent in electrical conductivity can be obtained. In addition, when the amount of the water-repellent resin is 2 parts by mass or less, electrical conductivity improves, and when it is 1 part by mass or less, electrical conductivity further improves. Here, the "fluorine / carbon intensity ratio" means the value of "mass of fluorine atoms" / "mass of carbon atoms" obtained when measuring the surface of the carbon fibers on the surface opposite to the surface on which the microporous layer is formed. Energy dispersive X-ray spectroscopy may be mentioned as a means for measuring the fluorine / carbon intensity ratio.

[0039] In addition, the porous carbon sheet of the present invention has a water sliding angle of 0° or more and 30° or less on at least one surface thereof. A small sliding angle means high water repellency. If the sliding angle of the surface is within the aforementioned range, sufficient water repellency is obtained, and when the fuel cell generates power, even if water vapor condenses into water droplets inside the gas diffusion electrode, the function of discharging the water droplets can be sufficiently exhibited. From this viewpoint, it is preferable that the water sliding angle on at least one surface of the porous carbon sheet is 30° or less.

[0040] As a method for water-repellent treatment of a porous carbon sheet, in addition to the generally known treatment technique of immersing the porous carbon sheet in a dispersion containing a water-repellent resin, coating techniques such as die coating and spray coating for applying the water-repellent resin to the porous carbon sheet are also applicable. Processing by a dry process such as sputtering of a water-repellent resin can also be applied. After the water-repellent treatment, a drying step and further a sintering step may be added as necessary.

[0041] The microporous layer according to the present invention is a layer having even finer pores than a porous carbon sheet, preferably with an average pore diameter of 0.01 to 10 μm. Since high conductivity is required when the gas diffusion electrode according to the present invention is used as an electrode, it is composed of conductive fine particles such as carbon black, carbon nanotubes, carbon nanofibers, graphene, and graphite.

[0042] Furthermore, the microporous layer is required to have properties such as conductivity, gas diffusion, water drainage, moisture retention, and thermal conductivity. In addition, when used on the anode side of a fuel cell, it is required to have strong acid resistance, and when used on the cathode side, it is required to have oxidation resistance. Therefore, in addition to conductive fine particles, it contains a water-repellent resin, including a fluororesin. Examples of fluororesins included in the microporous layer are the same as those preferred when water-repellent treating porous carbon sheets, such as PTFE, FEP, PFA, and ETFE. PTFE or FEP are preferred because they have particularly high water repellency. The content of the water-repellent resin in the microporous layer is preferably 1 to 70 parts by mass, and more preferably 5 to 60 parts by mass, per 100 parts by mass of conductive fine particles contained in the microporous layer. If the amount of water-repellent resin is 1 part by mass or more, the microporous layer will have excellent drainage and mechanical strength, which is preferable. On the other hand, if it is 70 parts by mass or less, the microporous layer will have excellent conductivity, which is preferable.

[0043] In this invention, the microporous layer that has seeped into the porous carbon sheet is hereinafter referred to as the seeped portion. The ratio of the thickness of the seeped portion to the thickness of the porous carbon sheet, which is set to 100%, is hereinafter referred to as the seeped amount, and in this invention, it is preferable that the seeped amount is 10% or more and 100% or less. By setting the seeped amount to 10% or more, the seeped microporous layer reinforces the porous carbon sheet, improving the springiness of the gas diffusion electrode.

[0044] A porosity of less than 80% in the microporous layer is preferable because it prevents a decrease in power generation performance due to drying (dry-up) of the electrolyte membrane. However, if the porosity is too low, gas diffusion is impaired, so it is preferable to set the lower limit at 60%. When such a gas diffusion electrode is incorporated into a fuel cell, dry-up is suppressed and power generation performance at high temperatures is improved.

[0045] The gas diffusion electrode of the present invention preferably has a pore diameter in the range of 0.03 μm to 1.00 μm, with the diameter of the pore having the largest volume (peak diameter) being in the range of 0.10 μm to 1.00 μm. If the peak diameter is 0.10 μm or more, high gas diffusion can be ensured even if the microporous layer is impregnated into the carbon fiber fabric. Furthermore, if the peak diameter is 1.00 μm or less, preferably 0.80 μm or less, flooding can be effectively suppressed.

[0046] The peak diameter is determined from the pore size distribution obtained by measuring at a measurement pressure of 6 kPa to 414 MPa (pore size of 30 nm to 400 μm) using the mercury intrusion method. If multiple peaks appear in a close pore size region, the peak diameter of the highest peak is adopted. As a measuring device, for example, Shimadzu Corporation's Autopore 9520 can be used. A gas diffusion electrode having a peak diameter in the range of 0.10 to 1.00 μm can be obtained by controlling the primary particle size and dispersion state of conductive fine particles contained in the microporous layer.

[0047] Furthermore, the gas diffusion electrode of the present invention has an in-plane air permeability of 100 μm when compressed from both surfaces at a pressure of 1 MPa. ３ 3,000 μm or more ３ Preferably, the in-plane air permeability is 100 μm. ３ When the surface is above this level, the gas diffuses sufficiently within the plane, improving the power generation performance of the fuel cell. Furthermore, the in-plane air permeability is 3,000 μm. ３ The following conditions allow for the creation of a porous carbon sheet with a good balance of mechanical strength.

[0048] Next, a preferred method for obtaining the gas diffusion electrode of the present invention will be specifically described. However, the present invention is not limited to the following description, and any description of preferred embodiments within each description can also be interpreted as a description of the present invention as a broader concept.

[0049] <Weaving Process> In the weaving process, textiles are produced using yarn spun from fibers made from natural cellulose (cotton, bamboo fiber, etc.), regenerated cellulose (rayon, acetate), polyacrylonitrile-based, pitch-based, polynosic-based, phenolic resin-based, poly(p-phenylene terephthalamide), or mixtures thereof as starting materials. Acrylic fibers, mainly composed of polyacrylonitrile, are desirable as starting materials because they are stable in terms of strength. As for the yarn, either long filaments or spun yarn made from short fibers may be used, but spun yarn made from short fibers is desirable because it is readily available and inexpensive. Examples of weaving methods include plain weave, twill weave, or satin weave.

[0050] <Flame-retardant process> The fabric obtained in the weaving process is heat-treated in air using a batch-type or continuous-type heating furnace. By heat-treating at 200-300°C, flame-retardant fabric is obtained. When flame-retardant fibers or carbon fibers are used as weaving materials, it is necessary to use relatively thick threads, and the electrical resistance in the thickness direction of the carbon fiber fabric remains high. In this invention, by first creating a thin fabric from acrylic fibers and then performing the flame-retardant process and carbonization process in a planar manner, the total energy cost can be reduced compared to weaving flame-retardant fibers or carbon fibers. In addition, since thin fibers can be used as raw materials, carbon fiber fabrics with excellent conductivity can be produced. On the other hand, conventionally, for example in Patent Document 1, carbon fiber fabrics are produced by going through the flame-retardant process as fibers, which has the problem of higher energy costs and equipment costs compared to directly flame-retarding the fabric.

[0051] <Carbonization Process> Carbon fiber fabrics can be produced by firing flame-resistant fabrics in an inert atmosphere. This firing can be carried out using either a batch-type or a continuous-type heating furnace. An inert atmosphere can be obtained by flowing an inert gas such as nitrogen gas or argon gas through the furnace.

[0052] The maximum firing temperature is preferably in the range of 800 to 3,000°C, and more preferably in the range of 1,100 to 1,500°C. A maximum temperature of 1,100°C or higher is preferable because it promotes carbonization of the flame-resistant fabric, resulting in a carbon fiber fabric with excellent conductivity and thermal conductivity. On the other hand, a maximum temperature of 1,500°C or lower is preferable because it reduces the operating cost of the heating furnace.

[0053] <Water-repellent treatment> Methods for water-repellent treatment of carbon fiber fabrics include immersing the carbon fiber fabric in a dispersion containing a water-repellent agent, or applying a dispersion containing a water-repellent agent to a porous carbon sheet using die coating, spray coating, etc. After the water-repellent treatment, a drying process and even a sintering process may be added as needed. The amount of water-repellent resin applied should be adjusted so that the fluorine / carbon strength ratio on the carbon fiber surface is between 0.10 and 0.30.

[0054] <Formation of Microporous Layer> An electrode formed by applying a microporous layer coating solution to one side of a carbon fiber fabric is suitably used as a gas diffusion electrode. The microporous layer coating solution may contain a dispersion medium such as water or an organic solvent, or a dispersion aid such as a surfactant. Water is preferred as the dispersion medium, and a nonionic surfactant is preferred as the dispersion aid. If conductive fine particles are included, a microporous layer with excellent conductivity can be obtained, and if a water-repellent resin is included, a microporous layer with excellent drainage properties for discharging water generated by electrochemical reactions in the fuel cell to a separator, and excellent mechanical strength can be obtained. Coating of the microporous layer coating solution to a porous carbon sheet can be carried out using various commercially available coating devices. As for coating methods, screen printing, rotary screen printing, spray atomization, intaglio printing, gravure printing, die coater coating, bar coating, and blade coating can be used, but die coater coating is preferred because it allows for quantitative determination of the coating amount regardless of the surface roughness of the porous carbon sheet. The coating methods exemplified above are merely examples and are not necessarily the only applicable methods.

[0055] <Membrane Electrode Assembly> In the present invention, a membrane electrode assembly can be formed by joining the above-mentioned gas diffusion electrode to at least one side of an electrolyte membrane having catalyst layers on both sides. Arranging the gas diffusion electrode so that the microporous layer side is in contact with the catalyst layer side is preferable because it facilitates back diffusion of the generated water, increases the contact area between the catalyst layer and the gas diffusion electrode, and reduces contact electrical resistance. Platinum is usually used as the catalyst for the catalyst layer. It is preferable to use a perfluorosulfonic acid-based polymer material with high proton conductivity, oxidation resistance, and heat resistance for the electrolyte membrane.

[0056] <Fuel Cell> A fuel cell is one aspect of the present invention. The fuel cell of the present invention is a fuel cell having the gas diffusion electrode of the present invention. That is, it refers to a fuel cell having separators on both sides of the membrane electrode assembly described above. The separator has a flow path to allow fuel gas to flow into the anode-side gas diffusion layer and oxidizing gas to flow into the cathode-side gas diffusion layer. The separator and the flow path can be of any shape that allows fuel gas or oxidizing gas to flow in and out. A fuel cell stack can be constructed by stacking multiple of the above fuel cells.

[0057] <Liquid Electrolysis Apparatus> The liquid electrolysis apparatus is one aspect of the present invention. The liquid electrolysis apparatus of the present invention has the gas diffusion electrode of the present invention. That is, it has a liquid electrolysis cell having separators on both sides of the membrane electrode assembly described above.

[0058] <Redox Flow Battery> The redox flow battery is one aspect of the present invention. The redox flow battery of the present invention uses the gas diffusion electrode of the present invention as the positive electrode and / or negative electrode. The porous carbon sheet of the present invention can be used as an electrode in either a flow-through type or a flow-by type cell.

[0059] <Mobile Vehicles> Mobile vehicles are one aspect of the present invention. The fuel cell in the present invention is a fuel cell installed in a mobile vehicle such as an automobile, ship, or railway, and can be used as a power source for said mobile vehicle. In other words, the mobile vehicle of the present invention refers to a mobile vehicle equipped with the fuel cell of the present invention.

[0060] The present invention will be specifically described below with reference to examples. However, the present invention is not limited in any way by the following examples.

[0061] <Preparation of Porous Carbon Sheet> A plain weave acrylic fabric was prepared using 100-meter count acrylic spun yarn as both the warp and weft. This acrylic fabric was heat-treated in a furnace with a maximum temperature of 250°C to produce a flame-resistant fabric. This flame-resistant fabric was fired in a furnace with a maximum temperature of 1,250°C, maintained in a nitrogen gas atmosphere, resulting in a basis weight of 40 g / m². ２ A carbon fiber fabric was obtained. The compression thickness of the carbon fiber fabric was measured using the compression test mode of the "Autograph®" AGS-X manufactured by Shimadzu Corporation. The carbon fiber fabric was cut to a size of 20 mm x 20 mm, sandwiched between smooth metal rigid electrodes, and the thickness of the carbon fiber fabric was measured when an average pressure of 1 MPa was applied, and the result was 76 μm.

[0062] Next, the carbon fiber fabric was immersed in a diluted aqueous solution of PTFE dispersion ("Polyflon®" D-210C; manufactured by Daikin Industries, Ltd.; containing 60 parts by mass of PTFE in the dispersion medium (water)) and then dried at 120°C.

[0063] <Fabrication of Gas Diffusion Electrode> Acetylene black "Denka Black" (registered trademark) (manufactured by Denki Kagaku Kogyo Co., Ltd.), water-repellent resin "Polyflon" (registered trademark) PTFE dispersion D-210C (manufactured by Daikin Industries, Ltd.), surfactant "TRITON" X-100 (manufactured by Nakalai Tesque Co., Ltd.), and purified water were mixed using a disperser in proportions of 7.0 parts by mass, 2.5 parts by mass, 14.1 parts by mass, and 76.4 parts by mass, respectively, to prepare a microporous layer coating solution. The microporous layer coating solution was applied to one side of a porous carbon sheet using a die coater and heated and dried at 120°C for 10 minutes. After heating and drying, it was sintered at 380°C for 3 minutes to produce a gas diffusion electrode having a microporous layer on the surface of the porous carbon sheet.

[0064] <Evaluation> [Fluorine / Carbon Intensity Ratio on Carbon Fiber Surface] Using SEM-EDX (energy-dispersive X-ray fluorescence), elemental mapping images of carbon and fluorine on the carbon fiber surface were obtained on the side of the gas diffusion electrode opposite to the side where the microporous layer was formed, under conditions of an acceleration voltage of 10 kV and a magnification of 200x. The X-ray dose (counts) corresponding to the mass of fluorine atoms and carbon atoms was quantified, and the fluorine / carbon intensity ratio was determined. This measurement was performed at 10 arbitrary points, and the average value was taken as the fluorine / carbon intensity ratio. For the SEM-EDX, a system consisting of a scanning electron microscope SEM H-3000 manufactured by Hitachi, Ltd., with an energy-dispersive X-ray fluorescence analyzer SEMEDEX Type-H attached was used.

[0065] [Sliding Angle] Using the sliding angle measurement mode of the DM501 automatic contact angle meter manufactured by Kyowa Interface Science Co., Ltd., a 10 μL water droplet was dropped onto the side of the gas diffusion electrode opposite to the side with the microporous layer. The sample stage was tilted in stages from a horizontal position (tilting at 1° / second, pausing for 1 second, and this was repeated), and the angle at which the water droplet slid off and disappeared from the measurement screen was defined as the sliding angle (degrees). This measurement was performed at 10 arbitrary points, and the average value was defined as the sliding angle. A smaller sliding angle value indicates a porous carbon sheet with higher water repellency.

[0066] [Diameter of carbon fibers] Using a microscope or other magnification equipment, five arbitrary points on the carbon fibers contained in the porous carbon sheet were measured under a magnification of 2,500 times, and the average value of these measurements was calculated as the diameter of the carbon fibers (μm).

[0067] [Number of carbon fibers in the warp and weft threads of carbon fiber fabric] When a porous carbon sheet is composed of carbon fiber fabric, the cross-sections of the warp and weft threads were examined at five locations each under magnification of 500 times using a microscope or other magnifying equipment. The average number of carbon fibers contained in each cross-section was calculated as the number of carbon fibers in the warp and weft threads. The cross-sections of the warp and weft threads were revealed by cutting the carbon fiber fabric with a sharp blade.

[0068] [Opening Ratio of Carbon Fiber Fabric] When a porous carbon sheet is composed of carbon fiber fabric, the warp pitch Tp, weft pitch Yp, warp width Tw, and weft width Yw were measured under magnification of 200 times using a microscope or other magnifying equipment, and the opening ratio was derived using the above formula (1). This measurement was performed at five arbitrary points, and the average value was taken as the opening ratio.

[0069] [Porrosion of the Microporous Layer] Using a Hitachi S-4800 scanning electron microscope, 20 different locations were randomly selected from the cross-section perpendicular to the surface of the gas diffusion electrode in the region of the microporous layer. These locations were magnified to approximately 20,000 times and photographed. The porosity was measured by binarizing the void and non-void areas in each image and calculating the average value. For the preparation of the cross-section of the gas diffusion electrode, an ion milling system IM4000 manufactured by Hitachi High-Technologies Corporation was used.

[0070] [Pore Peak Diameter] The pore diameter distribution of the gas diffusion electrode (distribution showing the pore volume relative to the pore diameter) was obtained by the mercury intrusion method. Three sample pieces of approximately 12 mm x 20 mm were cut from the gas diffusion electrode, weighed precisely, and placed in a measurement cell without overlapping. Mercury was then injected under reduced pressure. The measurement was then performed under the following conditions: • Measurement pressure range: Pressure of 6 kPa at the start of measurement (pore diameter 400 μm) to pressure of 414 MPa at the end of measurement (pore diameter 30 nm) • Measurement cell mode: Pressure increase process within the above pressure range • Cell volume: 5 cm³ ３・Surface tension of mercury: 485 dyn / cm ・Contact angle of mercury: 130° An Autopore 9520 manufactured by Shimadzu Corporation was used as the measuring device. The diameter of the pore having the largest volume (peak diameter) within the pore diameter range of 0.03 to 1.00 μm was determined from this pore size distribution. When a plurality of peaks appear in close pore diameter regions, the peak diameter of the highest peak was employed.

[0071] [Penetration Amount of Microporous Layer] First, a cross-section perpendicular to the surface (cross-section in the thickness direction) of the gas diffusion electrode was cut out using an ion milling apparatus (Model IM4000, manufactured by Hitachi High-Technologies Corporation), and observation was performed with a SEM (S-4800, manufactured by Hitachi, Ltd.) at an image magnification of 200 times. Next, a method for determining the penetration amount of the microporous layer 1 will be described with reference to FIG. 1. The distance between the microporous layer-side surface (10) of the carbon fiber fabric 2 and the porous carbon sheet-side surface (11) of the carbon fiber fabric 2 is defined as the thickness (Ta) of the carbon fiber fabric 2. Among the portions where the microporous layer 1 penetrates into the carbon fiber fabric 2 (penetrated portion), starting from the point (12) that penetrates most toward the porous carbon sheet-side surface of the carbon fiber fabric 2, a line parallel to the microporous layer-side surface (10) of the carbon fiber fabric 2 is drawn, and this line is defined as the innermost surface (13) of the microporous layer 1. The distance between the microporous layer-side surface (10) of the carbon fiber fabric 2 and the innermost surface (13) of the microporous layer 1 is defined as the thickness (Tb) of the penetrated portion of the microporous layer 1. Further, the penetration amount was obtained from the formula of (thickness of penetrated portion (Tb) / thickness of carbon fiber fabric 2 (Ta)) × 100 (Tb / Ta × 100). Three penetrated portions of the microporous layer 1 were defined from three images, and the penetration amount was obtained as the average value of the three values.

[0072] [In-Plane Air Permeability] A porous carbon sheet or gas diffusion electrode punched into a donut shape with an outer diameter of φ40 mm and an inner diameter of φ10 mm was pressed with a pressure of 1 MPa by a pressing device, air was flowed from the inner side with a mass flow controller, the pressure was measured when the flow rate was increased by 0.1 L / min each time, and the ratio of the change in air flow rate to pressure was obtained from the following formula (2). I = ΔQ / ΔP × μ × ln(r ０ / r ｉ ) / 2π ・・・(2) In formula (2), I is the in-plane air permeability (μm ３Q is the airflow rate (L / min), P is the pressure (kPa), and μ is the air viscosity (1.8 × 10⁻¹⁰). ―５ kg / (m・s)), r ０ The outer diameter of the sample (m), r ｉ The values ​​of and indicate the inner diameter (m) of the sample. ΔQ / ΔP represents the slope when the airflow rate and pressure are linearly approximated. A larger value for in-plane air permeability I indicates a porous carbon sheet or gas diffusion electrode that allows gas to diffuse more easily in the in-plane direction.

[0073] [Sprungness] The springiness of a porous carbon sheet or gas diffusion electrode was measured using the compression test mode of the "Autograph (registered trademark)" AGS-X manufactured by Shimadzu Corporation. The porous carbon sheet or gas diffusion electrode was cut to a size of 20 mm x 20 mm, sandwiched between smooth metal rigid electrodes, and an average pressure of 2.0 MPa was applied. After releasing the pressure once, it was repressurized and an average pressure of 1.5 MPa was applied, and the thickness (μm) of the porous carbon sheet or gas diffusion electrode was measured after 25 seconds. Then, the average pressure was reduced to 0.5 MPa, and the thickness of the gas diffusion electrode was measured after 25 seconds. The difference between the thickness of the porous carbon sheet or gas diffusion electrode when an average pressure of 0.5 MPa was applied and the thickness when an average pressure of 1.5 MPa was applied was defined as the springiness.

[0074] (Example 1) A gas diffusion electrode was prepared according to the above-described <Preparation of Porous Carbon Sheet> and <Preparation of Gas Diffusion Electrode>. The porous carbon sheet and gas diffusion electrode obtained by adjusting the amount of water-repellent resin applied so that the fluorine / carbon strength ratio on the carbon fiber surface was 0.30 or less were measured according to the above-described <Evaluation> for the fluorine / carbon strength ratio on the carbon fiber surface, the sliding angle, the diameter of the carbon fiber, the number of carbon fibers contained in the warp and weft of the carbon fiber fabric, the opening ratio of the carbon fiber fabric, the porosity of the microporous layer, the pore peak diameter, the amount of penetration, the in-plane air permeability, and the springiness. The results are shown in Table 1.

[0075]

[0076] (Example 2) In Example 2, the porous carbon sheet and gas diffusion electrode were fabricated and various measurements were performed in the same manner as in Example 1, except that the die coater was brought closer to the porous carbon sheet than in Example 1 to increase the amount of microporous layer penetration, and the microporous layer coating solution was applied. The results are shown in Table 1.

[0077] (Example 3) In Example 3, the porous carbon sheet and gas diffusion electrode were fabricated and various measurements were performed in the same manner as in Example 2, except that the die coater was brought closer to the porous carbon sheet than in Example 2 to coat it with the microporous layer coating solution in order to increase the amount of microporous layer penetration. The results are shown in Table 1.

[0078] (Comparative Example 1) In Comparative Example 1, the porous carbon sheet and gas diffusion electrode were prepared and various measurements were performed in the same manner as in Example 1, except that the amount of water-repellent resin applied was adjusted so that the fluorine / carbon strength ratio on the carbon fiber surface exceeded 0.30. The results are shown in Table 1.

[0079] (Comparative Example 2) In Comparative Example 2, instead of carbon fiber fabric, a porous carbon sheet with a basis weight of 40 g / m² was used. ２ A porous carbon sheet and gas diffusion electrode were fabricated and various measurements were performed in the same manner as in Example 3, except that carbon paper was used. The results are shown in Table 1.

[0080] As shown in Table 1, Examples 1-3 and Comparative Example 1 have a sliding angle of 14 degrees or less and excellent water repellency, and the in-plane air permeability of the gas diffusion electrode is 457 μm. ３ The results above indicate excellent gas diffusion properties. On the other hand, while the springiness of the porous carbon sheets in Examples 1 to 3 was 3.4 μm or more and the springiness of the gas diffusion electrodes was 4.6 μm or more, Comparative Example 1 had low springiness. Thus, it can be said that Examples 1 to 3 are porous carbon sheets and gas diffusion electrodes with superior springiness compared to Comparative Example 1.

[0081] Furthermore, compared to Comparative Example 2, Example 3 exhibits superior water repellency (slipperiness) in the porous carbon sheet and superior gas diffusivity (in-plane air permeability) in the gas diffusion electrode, while also having equivalent or better spring properties. Through this invention, we have been able to improve the spring properties, which were a challenge for carbon fiber fabrics, and obtain a gas diffusion electrode with excellent spring properties, drainage, and gas diffusivity.

[0082] The porous carbon sheet of the present invention can be suitably used as an electrode in fuel cells, liquid electrolytic devices, and redox flow batteries, and in particular as a gas diffusion electrode in polymer electrolyte fuel cells used as a power source for mobile devices such as fuel cell vehicles and ships.

[0083] 1. Microporous layer 2. Carbon fiber fabric 10. Surface of the carbon fiber fabric facing the microporous layer 11. Surface of the carbon fiber fabric facing the porous carbon sheet 12. Point in the permeation area where the material has penetrated the most towards the porous carbon sheet surface of the carbon fiber fabric 13. Innermost surface of the microporous layer

Claims

1. A porous carbon sheet having a water sliding angle of 0 degrees or more and 30 degrees or less on at least one surface, and a fluorine / carbon strength ratio of 0.10 or more and 0.30 or less.

2. The porous carbon sheet according to claim 1, wherein the porous carbon sheet is a carbon fiber felt or a carbon fiber fabric.

3. The porous carbon sheet according to claim 1, comprising carbon fibers having a diameter of 3 μm or more and 10 μm or less.

4. The porous carbon sheet according to claim 1, wherein the porous carbon sheet is made of a carbon fiber fabric and includes spun yarn in which the number of carbon fibers contained in the cross-section is 50 or more and 150 or less.

5. The porous carbon sheet according to claim 1, wherein the porous carbon sheet is made of a carbon fiber fabric, and the opening ratio of the carbon fiber fabric is 5% or more and 75% or less.

6. The porous carbon sheet according to claim 1, comprising 0.1 parts by mass or more and 3 parts by mass or less of fluororesin per 100 parts by mass of the porous carbon sheet.

7. A gas diffusion electrode comprising a microporous layer formed on one side of the porous carbon sheet according to claim 1, the layer being composed of a carbon-based filler and a fluororesin.

8. The gas diffusion electrode according to claim 7, characterized in that the amount of penetration, which is the ratio of the thickness of the portion of the microporous layer that has penetrated into the porous carbon sheet, is 10% or more and 100% or less, when the thickness of the porous carbon sheet, defined as the distance between the microporous layer side surface of the porous carbon sheet and the porous carbon sheet side surface of the porous carbon sheet, is 100%.

9. The gas diffusion electrode according to claim 7, wherein, within the range of pore diameters from 0.03 μm to 1.00 μm, the diameter of the pore with the largest volume (peak diameter) is within the range of 0.10 μm to 1.00 μm.

10. In-plane air permeability is 100 μm ３ The gas diffusion electrode according to claim 7.

11. A fuel cell having a gas diffusion electrode according to any one of claims 7 to 10.

12. A liquid electrolytic apparatus having a gas diffusion electrode according to any one of claims 7 to 10.

13. A redox flow battery having a gas diffusion electrode according to any one of claims 7 to 10.

14. A mobile body equipped with the fuel cell described in claim 11.

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